1. Field of the Invention
The present invention relates to a method for regenerating a particulate filter.
2. Description of the Related Art
Particulates or particulate matter in exhaust gas from a diesel engine is mainly constituted by carbonic soot and a soluble organic fraction of high-boiling hydrocarbons and contains a trace of sulfate or misty sulfuric acid fraction. In order to reduce a discharged amount of particulates from the engine, a particulate filter is conventionally employed and arranged in an exhaust pipe through which the exhaust gas flows.
The particulate filter of this kind is a porous honeycomb structure made of ceramics such as cordierite and having lattice-like compartmentalized passages; alternate ones of the passages have plugged inlets and the remaining passages with unplugged open inlets are plugged at their outlets. Thus, only the exhaust gas passing through the thin porous compartment walls is discharged downstream.
The particulates in the exhaust gas, which are captured and accumulated on the inner thin porous compartment walls of the particulate filter, require to be burned off so as to regenerate the particulate filter before exhaust resistance considerably increases due to clogging. However, the exhaust gas from the engine in a normal operation state rarely has a chance to reach a temperature level at which the particulates ignite by themselves. Then, a catalytic regeneration type particulate filter has been developed into practical use which integrally carries an oxidation catalyst made from alumina carrying platinum and added with a required amount of rare-earth element such as cerium.
Use of such catalytic regeneration type particulate filter facilitates oxidation of the captured and accumulated particulates and lowers their ignition temperature so that the particulates can be burned off even at an exhaust gas temperature lower than before.
However, even in use of such catalytic regeneration type particulate filter, a captured amount of particulates may exceed a burned-off amount of particulates in engine operation areas with low exhaust temperature level; and continued engine operation with such low exhaust temperature level may hinder good regeneration of the particulate filter, resulting in excessive accumulation of the captured particulates in the particulate filter. In order to overcome this, it has been envisaged that fuel is added to the exhaust gas upstream of the particulate filter through, for example, post fuel injection to forcibly regenerate the particulate filter before the particulates are substantially accumulated.
Specifically, the fuel added to the exhaust gas upstream of the particulate filter generates gaseous hydrocarbons which are oxidized on the oxidation catalyst carried by the filter to produce exothermic heat and raise the temperature of the carried catalyst to burn off the particulates, thus regenerating the particulate filter.
This kind of forcible regeneration of a particulate filter is disclosed, for example, in JP 2003-155915A and JP 2003-222040A.
Particularly in an exhaust purifier with a flow-through type oxidation catalyst upstream of a particulate filter to support oxidation of the captured particulates in the filter, gaseous hydrocarbons are oxidized on the flow-through type oxidation catalyst upstream of the particulate filter to generate exothermic heat and raise the temperature of the exhaust gas which is introduced into the particulate filter; as a result, the particulate filter can be forcibly regenerated at a further low exhaust temperature level.
However, in a case of a vehicle continuously operated in light-load engine operation areas with extremely low exhaust temperature level at which gaseous hydrocarbons cannot be oxidized on an oxidization catalyst, e.g., in a case of a city shuttle-bus which tends to travel on congested roads, rise in temperature of the carried catalyst of the particulate filter cannot be expected even in the combination of fuel addition with arrangement of a flow-through type oxidation catalyst upstream of the particulate filter. As a result, particulates may be extremely captured by and accumulated in the particulate filter, leading to increased exhaust pressure which adversely affects engine performance. The particulates accumulated in a large amount may be suddenly ignited, leading to fusion damage of the particulate filter.
The present invention was made in view of the above and has its object to provide a method for regenerating a particulate filter which positively prevents a catalytic regeneration type particulate filter from having excessively captured and accumulated particulates.